The invention relates to data communication systems using the commercial telephone network to establish connections between terminals and to transmit and receive data over such connections. One familiar example of such data communication systems is a facsimile exchange system using the commercial telephone network where a user at one terminal dials the telephone number of another terminal and connects with it through the automatic telephone exchange. In such a system it is desirable to confirm that the connection is with the correct terminal prior to transmitting data. In one known facsimile system the calling terminal transmits a carrier signal at a predetermined frequency to the called terminal, and the called terminal in response transmits an acknowledgement signal back to the calling terminal. This avoids transmitting to a location which does not have a facsimile set that is a part of the system, but does not help if a telephone connection has been established to the wrong facsimile set of the same system. There are no means in the known commercial facsimile system for preventing misconnection of this and certain other types, and one object of the invention is to provide a technique and means to help prevent such misconnections.
To this end, in one embodiment of the invention the calling terminal establishes a telephone connection with the called terminal, and in response the called terminal transmits an acknowledgement signal back to the calling terminal. In response to this acknowledgement signal, the calling terminal transmits to the called terminal two separate identification codes: the identification code of the calling terminal and the identification code of the called terminal. Upon receipt of these two separate identification codes, the called terminal stores the code of the calling terminal for future use and compares the code of the called terminal received from the calling terminal with its own code, i.e., with the code of the called terminal which is always stored at the called terminal. An unfavorable comparison means that the called terminal is not the one that was meant to be called by the calling terminal or that there is some malfunction in either terminal or in the telephone line. Hence, if the comparison is unfavorable, the system ends the telephone connection between these terminals. If the comparison at the called terminal is favorable the called terminal transmits, back to the calling terminal, both the identification code of the called terminal and the identification code of the calling terminal previously received by the called terminal and stored for future use in the called terminal. Upon receipt of the two codes transmitted from the called terminal, the calling terminal compares each with the corresponding identification code previously transmitted by it to the called terminal. In case of an unfavorable comparison with respect to either of the codes (lack of correspondance between the two compared codes of the calling terminal or the two compared codes of the called terminal), the system ends the telephone connection because this indicates that the connection is between the wrong terminals, or that there is a malfunction in at least one of the terminals, or in the telephone line. In case of a favorable comparison, the calling data terminal starts transmitting data to the called data terminal. The data may be broken into blocks, and the called terminal may transmit back to the calling terminal an acknowledgement signal after the end of each block to indicate that the block has been received and that the called terminal is still in order. The calling terminal then sends another block of data to the called terminal, etc., until there are no more data blocks, in which case the system ends the telephone connection between these terminals.
In a somewhat different embodiment of the invention, after the telephone connection has been established between the calling terminal and the called terminal, the called terminal transmits an acknowledgement signal back to the calling terminal. In response to this acknowledgement signal the calling terminal transmits to the called terminal the identification code of the called terminal. In response to this, the called terminal compares the identification code received from the calling terminal with its own identification code, which is always stored in it. In case of an unfavorable comparison, the system ends the telephone connection between these terminals. In case of a favorable comparison, the called terminal again sends an acknowledgement signal to the calling terminal. If the calling terminal does not receive this second acknowledgement signal, for whatever reason, the calling terminal ends the telephone connection. If the calling terminal receives this acknowledgement signal, then it starts transmitting data to the called terminal. The data may again be in blocks, with an acknowledgement signal transmitted from the called terminal to the calling terminal after each block of data has been received by the called terminal.
In a still different embodiment of the invention, the calling terminal establishes the telephone connection with the called terminal, and in response to this the called terminal transmits an acknowledgement signal back to the calling terminal and then unconditionally transmits back to the calling terminal the identification code of the called terminal, which is always stored in that called terminal. The calling terminal compares the identification code received from the called terminal with the identification code of that same called terminal which has been previously stored in the calling terminal. If the comparison is unfavorable, the system ends this telephone connection. If the comparison is favorable, the calling terminal starts transmitting a block of data. At the end of the first, and any subsequent, block of data the called terminal sends back to the calling terminal an acknowledgement signal.
The terminals described here can operate either in a calling or in a called mode, and the confirmation procedure is fully automatic. The only operation which may be manual is the initial establishing of a telephone connection by the calling terminal.